1. Field of the Invention
The present invention relates to a linear-compressor control system, to the respective control method, and to the linear control incorporating the control system of the present invention.
2. Description of the Prior Art
The basic objective of a cooling system is to keep a low temperature inside one (or more) compartment(s) (or even closed environments, in the cases of air-conditioning systems), making use of devices that transport heat from the inside of said compartment(s) to the external environment, taking advantage of the measurement of the temperature inside this (these) environment(s) to control the devices responsible for transporting heat, seeking to maintain the temperature within pre-established limits for the type of cooling system in question.
Depending on the complexity of the cooling system and on the type of application, the temperature limits to be kept are more or less restricted.
A common form of transporting heat from the interior of a cooling system to the external environment is to use an airtight compressor connected to a closed circuit, which includes an evaporator and a condenser through which a cooling fluid circulates, this compressor having the function of promoting the cooled gas flow inside this cooling system, being capable of imposing a determined difference in pressure between the points where evaporation and condensation of the cooling gas occur, enabling the heat-transport process and the creation of a low temperature to take place.
Compressors are dimensioned so as to have the capacity of cooling higher than that necessary in a normal operation situation, critical demand situations being foreseen, wherein some type of modulation of the cooling capacity of this compressor is necessary to keep the temperature inside the cabinet within acceptable limits.
The most common form of modulating the cooling capacity of a conventional compressor is to turn it on and off, according to the temperature inside the cooled environment, taking advantage of the thermostat, which switches on the compressor when the temperature in the cooled room rises above the pre-established limit and switches it off when the temperature inside this environment has reached an equally pre-established lower limit, these limits being established in such a way that the pressures will equalize. Such a phenomenon can be observed in FIGS. 1 and 2. As disclosed therein, the average temperature TM oscillates, and the compressor is turned on and off when ever the temperature measured at a determined instant is above the desired level. The variation of the cooling fluid pressure can be observed in FIG. 2; it can be noted that the condensation pressure PC jumps significantly up and, at the same time, the evaporation pressure PE is reduced because of the loss of heat of the gas in the evaporator. Once the compressor has been turned off, the condensation pressure PC drops and the evaporation pressure PE rises, until they equalize, that is to say, until they are equal. The equalization of the condensation pressure PC and the evaporation pressure PE occurs because the cooling fluid, which was impelled by the (now off) compressor, spreads through the tubing until the pressure becomes equal at all the points.
For compressors having variable capacity, the control is performed by changing the compressor's rotation, that it to say, when the temperature of the cooled environment rises above a certain pre-established limit, the thermostat installed within the cooling system commands the compressor to raise rotation and, as a result, the capacity also rises until the temperature returns to the previous state, the moment when the rotation is decreased. However, for constructive reasons, there is a limit for the minimum rotation, and, if it is necessary to decrease the rotation to values lower than the minimum rotation, it will be necessary to turn off the compressor.
The behavior of a compressor having variable capacity can be observed in FIGS. 3 and 4, the variation in behavior of the condensation pressure PC and of the evaporation pressure PE in function of the average temperature TM being analogous to that of a conventional compressor, that is to say, once the compressor has been turned off, the condensation PC and the evaporation pressures PE equalize.
In the case of a linear compressor having variable capacity, the capacity is controlled by varying the volume displaced by the piston. This control is given by a signal from the thermostat installed within the cooling system, which commands the compressor to raise capacity (displaced volume) until the temperature returns to the previous state and again the displaced volume is diminished.
According to the teachings of the prior art, the control of the capacity of a conventional compressor presents problems due to the characteristics intrinsic in this type of equipment. As it is well known, in practice one does not manage to start a conventional compressor without the pressures of the cooling being equalized. This is because, in order for a conventional compressor to be started with non-equalized pressures, one has to use a high-torque starting motor which is too expensive, in addition to the problems with excessively high starting current which makes it unfeasible for this type of application. In this regard, one observes that a function of a variable-capacity compressor is exactly to prevent the pressures of the system from becoming unequalized, in order to prevent the need to stop the equipment for allowing the cooling-fluid pressures to remain equalized.
The result of this characteristic is that the compressor should work for long periods (within range of minutes) and be kept off for long periods as well (within range of minutes), in order to guarantee, at the same time, that the environment will reach the desired temperature and the cooling-fluid pressures will become equalized while the compressor is off, and the latter can be started again.
Another problem resulting from the use of compressors (be they of the variable-capacity type or common type) lies in the fact that, when the equipment is turned off, the fluid backflow inside the cooling circuit results in a loss of heat, since the pressure of the fluid compressed by the compressor will disperse or equalize with the rest of the pressure of the cooling circuit.
In addition to this drawback, compressors still have the problem of generating noise at the start, further requiring high electric starting current, which results in a higher consumption of electricity.
Since conventional compressors have the same characteristics, the knowledge of the present invention can be applied to rotary compressors that have application in domestic cooling systems and chiefly in air-conditioning systems.
When one makes use of a linear compressor, the capacity is altered, whereby the dead volume of the compressor (smaller displacement) is increased. This process causes the capacity to decrease and, as a result, there is a decrease in the efficiency of the compressor, caused by the increase in dead volume. In systems that operate with a low frequency (feed network frequency), there is still an additional loss due to the fact that the compressor undergoes a variation of its mechanical resonance frequency. In order to minimize this effect in systems with a fixed frequency, the compressor is adjusted to operate at the minimum capacity at a determined evaporation and condensation (optimum for this condition). Since the frequency is fixed and the compressor capacity is varied from the minimum to the maximum, the optimum functioning point also changes and the compressor loses approximately from 11 to 15% in efficiency.